optimized for loop & corrected comments (trekhleb#617)

The existing insertion sort implementation began by iterating from 0 until the end of the array, but it is only necessary to iterate from 1 until the end of the array, since at the 0th index, there is nothing to compare to the left of the element. In order to complete this change, I also had to update the tests to reflect the fact that the algorithm visits each index 1 less time. Finally, I corrected the grammar/wording of the comments. Co-authored-by: Oleksii Trekhleb <trehleb@gmail.com>

Author: austintheriot

src/algorithms/sorting/insertion-sort/InsertionSort.js

@@ -5,14 +5,14 @@ export default class InsertionSort extends Sort {
  const array = [...originalArray];
 
  // Go through all array elements...
- for (let i = 0; i < array.length; i += 1) {
+ for (let i = 1; i < array.length; i += 1) {
  let currentIndex = i;
 
  // Call visiting callback.
  this.callbacks.visitingCallback(array[i]);
 
- // Go and check if previous elements and greater then current one.
- // If this is the case then swap that elements.
+ // Check if previous element is greater than current element.
+ // If so, swap the two elements.
  while (
  array[currentIndex - 1] !== undefined
  && this.comparator.lessThan(array[currentIndex], array[currentIndex - 1])

src/algorithms/sorting/insertion-sort/__test__/InsertionSort.test.js

@@ -8,10 +8,10 @@ import {
 } from '../../SortTester';
 
 // Complexity constants.
-const SORTED_ARRAY_VISITING_COUNT = 20;
-const NOT_SORTED_ARRAY_VISITING_COUNT = 101;
-const REVERSE_SORTED_ARRAY_VISITING_COUNT = 210;
-const EQUAL_ARRAY_VISITING_COUNT = 20;
+const SORTED_ARRAY_VISITING_COUNT = 19;
+const NOT_SORTED_ARRAY_VISITING_COUNT = 100;
+const REVERSE_SORTED_ARRAY_VISITING_COUNT = 209;
+const EQUAL_ARRAY_VISITING_COUNT = 19;
 
 describe('InsertionSort', () => {
  it('should sort array', () => {